One-time programmable (“OTP”) circuits are widely used in many types of integrated circuits such as analog, digital, or mixed signal integrated circuits. They are used to precisely trim bias circuits such as bandgap reference circuits, generic internal voltage circuits or current references, as well as controlling other generic circuit parameters such as offset, gain, delay, and the like.
Typical OTP circuits include polysilicon fuse based circuits used with or without a separate pad for blowing the fuse using external components, metal fuse based circuits, circuits that use the complete breakdown of the base-emitter junction of an NPN bipolar transistor (bipolar junction transistor or “BJT”), circuits that use the breakdown of a thin-oxide layer, as well as EEPROM fuse cell based circuits.
The disadvantages of these circuits include large integrated circuit layout area as well as high fuse blowing currents (polysilicon and metal fuse based circuits) or high fuse blowing voltages (thin oxide and EEPROM based circuits). Additional disadvantages include one or two additional mask layers that are not normally used, resulting in additional manufacturing cost.
Polysilicon based circuits therefore need high currents for fuse blowing, and need a dedicated pad or a large MOS switch to blow the fuse. The fuse must be blown rapidly (within 10 to 100 ns) or it may easily fail to blow or recombine. Polysilicon based circuits need redundancy because of possible recombination, and may need many additional circuits for testing the overall quality of the fuse blowing.
Metal fuse based circuits also need high currents for fuse blowing, and also need a dedicated pad or large MOS switch for blowing the fuse. The fuse blowing must also be done rapidly or the fuse may fail to blow or will recombine. Redundant fuses are usually used with metal fuse based circuits. Additionally, metal fuses are not preferred because these fuses may destroy the oxide layer above the fuse by opening a hole in the oxide layer.
NPN bipolar transistor based circuits need a medium to high current for blowing the fuse, which is dependent upon the process used. However, a considerably high voltage is needed to blow the fuse, which is also process dependent. These types of circuits are usually preferred over polysilicon and metal based fuse blowing circuits.
EEPROM based circuits need a high voltage in order to trap charge on an MOS gate. One or two additional mask layers are needed, which adds unnecessary expense.
Thin oxide breakdown based circuits need high voltages to blow the fuse and usually take a relatively large integrated circuit die area, which also adds unnecessary expense.
What is desired, therefore, is an OTP circuit that is smaller than the prior art circuits mentioned above, that uses small fuse blowing currents and voltages, and is available in a standard BiCMOS process technology.